Wireless communication systems are constantly being pushed to accommodate the conflicting goals of higher data rates per user, and improved coverage area. A primary way to meet both of these desirable goals is by deploying more infrastructures (connected into the wired backbone, and eventually the PSTN and/or Internet). This infrastructure is generally quite expensive, in particular the base stations that comprise most of the infrastructure in cellular networks.
Two-tier networks, comprising a conventional cellular network overlaid with shorter range hotspots (e.g. femtocells, distributed antennas, or wired relays), offer an economically viable way to improve cellular system capacity. Femtocells are recently attracting interest for increasing overall system capacity and coverage, particularly for subscribers who are at home or in another common location. Femtocells are small virtual base stations that are usually deployed by the end user (perhaps with subsidy or logistical help from the service provider). This can result in a win-win: the subscriber gets high speed, reliable access at their most common locations (many subscribers currently complain about their service experience at home), and the service provider unloads considerable traffic off their expensive large-scale network. Because this results in two spatially overlaid networks (Base Stations being tier 1 and femtocells being tier 2), the composite network is often referred to as a “two-tier” network.
Femtocells, also known as home base stations (BTSs) or access point base stations, can connect to a service provider's network via a broadband backhaul connection (such as digital subscriber lines (DSL), cable, or even a radio link). Femtocells can allow service providers to extend service coverage indoors, where access would otherwise be limited or unavailable. Femtocells can incorporate the functionality of a typical base station while allowing for a simpler, self contained deployment.
However, interference between femtocells and macrocells in such networks can be a capacity-limiting factor if the femtocells and macrocells share the same spectrum. The cross-tier interference between macrocells and femtocells can suffocate the capacity due to the near-far problem, so in practice hotspots would typically want to use a different frequency channel than the potentially nearby high-power macrocell users. Centralized or coordinated frequency planning, which is difficult and inefficient, even in conventional cellular networks, is even more difficult, in a two-tier network. Alternatively, using expensive wireless spectrum, to coordinate between the cellular network and hotspots may be self-defeating as it, undermines the principle argument low capital and operating expenditures—for deploying femtocells in the first place.
On the other hand, femtocells and other types of supplemental infrastructure are likely to be deployed either randomly by users of the cellular network, or on an as-needed basis by the sendee provider. Allocating dedicated spectrum just for these devices and the mobile stations (MSs) interacting with them is highly undesirable since they may be sparse in many areas, rolled out slowly, and the demands on the available spectrum are intense, which is largely what motivates these hotspots in the first place. Therefore, methods and techniques that achieve frequency reuse between the two tiers are highly desirable.
In a shared spectrum two-tier network, near-far effects arising from cross-tier inference can create problems due to conventional signal strength based power control and can be particularly severe in a “closed access” deployment, where a femtocell allows only licensed subscribers to communicate with it. The worst-case scenario arises either when a high powered macrocell user on the cell edge causes interference to nearby femtocells, or when cell interior femtocell users [resp. femtocell BSs] create unacceptable interference to the macrocell base station [resp. nearby cellular users].